Rule reminder
When a perception verb takes a noun object + infinitive, the noun can be replaced by a clitic pronoun placed before the conjugated perception verb. The past participle then agrees in gender and number with the pronoun: 'Ho visto Maria uscire' → 'L'ho vista uscire'. With 'si' (reflexive/impersonal), special rules apply.
In the sentence 'L'ho vista passare', what does 'l'' refer to and what is the past participle agreement?